There are eight American turkeys painted on the walls of Schleswig's Cathedral of St Peter - which is odd... since the frescoes were created two centuries before Columbus even crossed the Atlantic.    How did the creatures come to be added to the medieval Biblical scene? Was this proof that the Germans reached the Americas before Columbus? Or do the painted birds tell a different story all together?  For a full list of sources used in this episode visit Tim Harford.com  See omnystudio.com/listener for privacy information.